EP 0 844 383 relates to a high pressure fuel injector for an internal combustion engine. The fuel injector has an injection nozzle defining a bore. The bore provides a flow path for high-pressure fuel between a fuel inlet and a plurality of outlets, the fuel being received from a high-pressure fuel supply passage. The fuel injector includes a needle which is slidable within the bore. At the lower end of the bore a needle seating is defined, the needle being engageable with the seating. The outlets are provided downstream of the seating so that, when the needle is engaged with the seating, fuel is prevented from being injected. When the needle is lifted from the seating, fuel is able to flow past the seating through the outlets and into an associated combustion chamber of the engine.
The needle includes at least one downstream-facing thrust surface against which high-pressure fuel in the bore acts to provide a lifting force to the needle. A control chamber is provided in the injection nozzle at an upper end of the needle, so that the upper end of the needle is exposed to fuel pressure in the control chamber. The control chamber receives fuel at high pressure from the supply passage, and is connectable to a low-pressure drain by way of a valve. The valve therefore controls the pressure of fuel in the control chamber, and hence determines the downward closing force acting on the upper end of the needle. In this way, the direction of the net hydraulic force acting on the needle, and hence the opening and closing movement of the valve needle, can be controlled.
A restriction, in the form of a small radial clearance between the valve needle and a portion of the bore, is provided for restricting the flow of fuel through the bore between the fuel inlet and the outlets. The restriction is upstream of the downstream-facing thrust surface. The restriction therefore ensures that, when the needle is open to allow injection and communication between the control chamber and drain is then closed to initiate closing of the needle, the upward force acting on the downstream-facing thrust surface due to fuel pressure in the bore is less than the downward force acting on the upper end of the needle due to fuel pressure in the control chamber. The pressure differential that results from the restriction gives rise to a substantial net closing hydraulic force on the needle, and allows for a fast needle closure to be achieved.
Providing a restriction within a fuel injector in order to generate a pressure drop between the high-pressure fuel supply and the injecting end of the injection nozzle, in an arrangement similar to that described above, is well known. There are various other ways in which a restriction can be provided in order to induce such a pressure drop. For example, the restriction can be provided near an injecting end of the nozzle, or alternatively within the high-pressure fuel passage that supplies the bore, downstream of where the high-pressure fuel passage supplies the control chamber.
U.S. Pat. No. 6,499,467, for example, discloses an arrangement in which the restriction takes the form of an orifice through a piston-type needle guide portion of the valve needle. The needle guide portion is situated near the injecting end of the nozzle and is remote from the control chamber. EP 0 971 118 discloses an arrangement in which the restriction is defined between an annular collar carried on the valve needle and the wall of the bore.
In all of these arrangements, the control chamber and the bore of the injector are fed from the same high-pressure fuel supply passage. However, the restriction ensures that, when needle closure is required, the closing force arising from the fuel pressure in the control chamber is sufficient to overcome the counteracting opening force arising from the fuel pressure in the bore, downstream of the restriction, acting on the downstream-facing thrust surface or surfaces of the needle.
A possible disadvantage of known arrangements such as those described above is that a relatively large drop in pressure occurs across the restriction. In practice this means that the injection pressure is lower than the pressure of fuel supplied to the injector. Hence, energy is wasted pumping the fuel to a higher pressure than is available for injection. It would be desirable to provide an arrangement in which a large pressure drop across a restriction is not required for operation of the injector, so that, for a given fuel supply pressure, a higher injection pressure can be achieved.
A further possible disadvantage of known injectors using restrictions in the aforementioned manner is that, because the bore of the injection nozzle is very small, the machining required to provide accurate radial distances for providing the desired pressure drop has to be very accurate. Such accuracy, particularly on such small scales, means that such injectors are both time consuming and costly to manufacture. It would be desirable to provide an injector which is cheaper and simpler to manufacture.
In these prior art arrangements, the rate of fuel through the restrictions is sensitive to the viscosity, and hence the temperature, of the fuel. In use, the temperature of the fuel varies considerably over the operating phases of the engine, which can result in unpredictable needle behaviour. Accordingly, it would be desirable to provide an injector which is less sensitive to fuel viscosity.
It is therefore an object of embodiments of the invention to at least partially mitigate one or more of the above mentioned problems.